By their nature, multi-cylinder marine engines used in outboard motors are compact. This means that the air induction and exhaust systems for these marine engines are not symmetric with respect to the various cylinders in the engine. For this reason as well as others, the operating conditions for the specific cylinders in the two-cycle engine vary greatly. For instance, differences in air motion within the cylinders can substantially affect the amount and quality of air/fuel mixing prior to combustion. Also, due to the different lengths of air intake paths and exhaust tuning paths, the effectiveness of the scavenging process among the cylinders varies, thus changing the quality of preparation in the cylinder before combustion. Further, exhaust and/or induction tuning effects can substantially alter the air/fuel ratio among the various cylinders (e.g. more air is provided to cylinders with relatively greater pressure pulse supercharging). All of these factors substantially alter combustion characteristics from cylinder to cylinder. It is difficult to account for these differences, especially over a wide range of operating conditions.
In addition, cylinder wall temperatures also tend to vary because it is difficult to provide an engine cooling system that maintains all of the cylinder wall temperatures at the same or substantially the same temperature. This is especially difficult because the combustion characteristics of the various cylinders provide different amounts of heat and these variations change with engine speed and load.
In direct fuel injection engines (i.e. fuel injection through the cylinder head), it is known to adjust fuel injection time and quantity on a cylinder-specific basis to account for some of the differences among combustion characteristics for the various cylinders. However, adjustments to fuel timing and quantity on a cylinder-specific basis can have limited affect on improving engine running quality, emissions, etc., especially in direct fuel injected engines. The significance of adjustments to fuel timing and quantity in direct fuel injected engines can be restricted because combustion ignition is sensitive in direct fuel injected engines, and ignition timing and dwell times need to be coordinated with fuel injection to ensure adequate ignition. In direct fuel injected engines, a spray of fuel is injected directly into the combustion chamber and is directed at least in part over the spark plug electrodes. At idle and light loads, the air/fuel mixture in the combustion chamber is dramatically stratified. In other words, the fuel/air mixture within the fuel spray from the fuel injector is fuel rich, whereas the mixture in the remaining portions of the combustion chamber contains virtually no fuel at all. At idle and light loads, it is therefore important that spark ignition occur at a moment in time when there is an appropriate air to fuel ratio in the vicinity of the spark plug electrodes, otherwise combustion misfire is possible. As engine speed and load increases, it is necessary to begin fuel injection into the combustion chamber earlier in the cycle, and this coupled with increased air motion in the cylinder causes mixing of air and fuel throughout the combustion chamber. At high engine speeds and load, the air and fuel are thoroughly mixed to produce a homogeneous and nearly stoichiometric blend in the combustion chamber which is easy to ignite. However, in the transition region (i.e. speeds above idle and light loads but below relatively high engine speeds and loads), the charge may be incompletely mixed in the combustion chamber. It is also likely that the air/fuel ratio in the chamber will be lean. Both of these characteristics in the transition region make ignition difficult unless fuel injection is properly timed with spark ignition.